PROJECT SUMMARY The long-term objective of this proposal is to identify mechanism(s) that promote clonal hematopoiesis of indeterminate potential (CHIP). CHIP is a risk factor for cardiovascular disease, myeloid hematological malignancy, and all-cause mortality. CHIP is thought to arise from mutant hematopoietic stem cells (HSC) carrying oncogenic mutations that endow the cells with increased fitness, leading to expansion of the mutant clone. Rare hematopoietic clones carrying CHIP-associated mutations are near-ubiquitous in healthy individuals. However, CHIP is largely confined in older individuals or patients with a history of smoking, chemo- or radiotherapy exposure. This suggests that physiological perturbation(s) unique to aging and genotoxin exposure, such as chronic inflammation, are required to drive CHIP. To better understand the mechanism underlying CHIP, we have conducted mouse studies that indicate chronic IL-1 production in the BM is a common consequence of aging and exposure to radiation or chemotherapy. Our preliminary data show that chronic IL-1 activates a cell growth arrest program associated with PU.1 induction in long-term HSC (HSCLT). Strikingly, Tet2-deficient HSCLT fail to fully activate this growth arrest program during IL-1 exposure. Along these lines, our data show that increased Tet2-deficient clonal expansion requires chronic IL-1. These preliminary data suggest that clonal expansion of mutant HSC is an emergent feature dependent on chronic inflammation. The studies proposed here will identify and characterize the molecular and cellular mechanisms by which chronic IL-1 promotes mutant HSC clonal expansion, using Tet2-deficiency as a model. Lines of investigation will include molecular and cellular analyses of normal and Tet2-deficient HSC exposed to IL-1, and competitive transplant assays to assess the functional impact of chronic IL-1 on normal and Tet2-deficient HSC fitness side-by-side. Lastly, experiments will assess whether IL-1 blockade can restore normal HSC fitness and reverse or limit clonal expansion. Altogether, our investigations could provide a basis for redefining CHIP as a potentially reversible process of somatic evolution in which an inflammatory BM environment selects for mutant HSC clones.